An exit-pupil expander is sometimes used in a scanned-beam display system to enlarge the aperture within which a viewer's eyes can perceive visual information, such as an image, generated by the system. An example of such a system operable as a head-mounted display is disclosed in U.S. Pat. No. 6,768,588, which is incorporated by reference.
One type of exit-pupil expander includes a diffractive optical element, such as a diffraction grating, which effectively splits an input light beam into multiple output beamlets that together define an output-beam envelope having a larger aperture than the input beam. As long as a viewer's eye is aligned with at least one of the beamlets, the viewer can substantially perceive the visual information that the input beam carries.
But unfortunately, this diffractive type of exit-pupil expander may have characteristics that are undesirable. For example, the output beamlets may have non-uniform brightness levels, particularly if the input light beam contains a wavelength different from the wavelength for which the exit-pupil expander is designed. Furthermore, for a color scanned-beam display system, the aperture of the output-beamlet envelope may exhibit relatively significant dependence on the wavelength or wavelengths of light within the input light beam. For example, for a color input beam that includes red (R), green (G), and blue (B) light, the expanded exit pupil may include overlapping R, G, and B output-beamlet envelopes having apertures of different sizes. Furthermore, a diffractive-type exit-pupil expander may make a scanned-beam display system susceptible to speckle when one or more coherent light sources are used.